Conventionally, as a device to obtain a magnetic profile of an object to be measured, a magnetic force microscope (MFM) is known. MFM includes ones to observe a direct current magnetic field (DC magnetic field) and ones to observe an alternating-current magnetic field (AC magnetic field). Since the present invention is a technique related to MFM to observe an alternating-current magnetic field, a conventional technique of MFM to observe an alternating-current magnetic field will be hereinafter described.
FIG. 10 is a figure to explain a conventional magnetic profile measuring device 8 using a MFM (see Patent Document 1). The magnetic profile measuring device 8 includes a cantilever 81, and a magnetized probe 811 is provided on a tip of the cantilever 81.
This magnetic profile measuring device 8 can measure a magnetic profile (a state of magnetic field distribution) of a space where an alternating-current magnetic field generated by an alternating-current magnetic field generating device 88 exists. Also, the magnetic profile measuring device 8 can measure a magnetic profile of the alternating-current magnetic field generating device 88 itself (for example, magnetic property of a surface of a writing head of a hard disc drive).
In FIG. 10, the cantilever 81 is driven by a driver 82 at a resonant frequency of the cantilever or at a frequency close to the resonant frequency of the cantilever.
For example, driven frequency of the cantilever 81 can be around the resonant frequency of the cantilever 81 (for example, around 300 kHz). Here, a frequency of the alternating-current magnetic field generated by the alternating-current magnetic field generating device 88 can also be around the resonant frequency of the cantilever 81, in the same way as the driven frequency of the cantilever 81.
When the alternating-current magnetic field is applied to the cantilever 81, the probe 811 formed on the tip of the cantilever 81 is directly subjected to a magnetic driving force.
This magnetic driving force directly changes amplitude and a phase of vibration of the cantilever 81 mechanically driven by the driver 82.
Therefore, when the driving frequency of the cantilever 81 is constant, the vibration of the probe 811 is modulated by both amplitude and phase, by the direct magnetic driving force from the alternating-current magnetic field.
It is possible to demodulate a magnetic signal corresponding to the magnetic force occurring between the probe 811 and the alternating-current magnetic field generating device 88 by detecting, for example optically, this vibration modulated by amplitude and phase.
A scanning mechanism 85 can scan the space where the alternating-current magnetic field exists by means of the probe 811. This makes it possible for the magnetic profile measuring device 8 to obtain a magnetic profile of the alternating-current magnetic field generating device 88 as an image of magnetic field distribution.
Referring to FIG. 10, an example in which the cantilever 81 is driven by the driver 82 at a resonant frequency of the cantilever or at a frequency close to the resonant frequency of the cantilever has been described. The frequency of the alternating-current magnetic field generated by the alternating-current magnetic field generating device 88 may be substantially different from the resonant frequency of the cantilever.
For example, when the resonant frequency of the cantilever 81 is around 300 kHz, it is possible to obtain the above described image of magnetic field distribution as long as the frequency of the alternating-current magnetic field generated by the alternating-current magnetic field generating device 81 is within the range of 1 to 100 kHz.
That is, when the probe 811 formed on the tip of the cantilever 81 is placed in the alternating-current magnetic field generated by the alternating-current magnetic field generating device 88, a non-resonant alternating-current magnetic force occurs between the probe 811 and the alternating-current magnetic field generating device 88.
However, this non-resonant alternating magnetic force cannot drive the cantilever 81 by itself, and therefore any signal of the alternating-current magnetic field cannot be obtained.
It is possible to detect the signal of the alternating-current magnetic field by driving the cantilever 81 by means of the driver 82 such as a piezoelectric element at a frequency around the resonant frequency of the cantilever while causing the non-resonant alternating magnetic force between the probe 811 and the alternating-current magnetic field generating device 88.
Meanwhile, when the cantilever 81 is driven at a frequency around the resonant frequency of the cantilever while causing the non-resonant alternating magnetic force between the probe 811 and the alternating-current magnetic field generating device 88, the cantilever 81 behaves as if its effective spring constant had been periodically changed. This apparent periodical change in the spring constant causes frequency modulation in the vibration of the cantilever 81.
The vibration of the cantilever 81 modulated by frequency can be detected for example optically by means of a vibration sensor 83 including a laser and a photodiode (PD). An alternating magnetic force signal demodulator 84 can take in the detection signal and demodulate the signal of the alternating magnetic force.
Since the scanning mechanism 85 can scan the space where the alternating-current magnetic field exists by means of the probe 811, the magnetic profile measuring device 8 can obtain the magnetic profile of the alternating-current magnetic field generating device 88 as an image of magnetic field distribution.
On the other hand, when the cantilever 81 is driven at a frequency different from the resonant frequency of the cantilever by means of a piezoelectric element or the like, amplitude modulation as well as frequency modulation occurs in the vibration of the cantilever 81.
By detecting for example optically this vibration modulated by frequency and amplitude to demodulate by frequency or by amplitude, it is possible to obtain a magnetic profile of the alternating-current magnetic field generating device as an image of magnetic field distribution.
As in the example described above, it is supposed that the frequency of the alternating-current magnetic field generated by the alternating-current magnetic field generating device 88 is greatly different from the resonant frequency of the cantilever 81 and a non-resonant alternating magnetic force occurs between the cantilever 81 and the alternating-current magnetic field generating device 88 in measuring the magnetic profile of the alternating-current magnetic field generating device 88. For example, when the alternating-current magnetic field generating device 88 includes a magnetic coil 881 and a signal generator 882, its magnetic profile is an alternating-current magnetic field generated by the magnetic coil 881.
When a phase of the alternating-current magnetic field is synchronized with (identical to) a phase of the signal generator 882, a lock-in amplifier 86 can obtain a magnetic field component perpendicular to a reference surface (for example, in a magnetic recording head of a hard disk drive, a sliding surface of the magnetic recording head) of the magnetic coil 88 (perpendicular magnetic field component Hp), referring to the synchronizing signal output from the signal generator 882.
In this case, the magnetic coil 881 of the alternating-current magnetic field generating device 88, if there is no delay in magnetization process of its magnetic material, generates the maximum perpendicular magnetic field when instantaneous value (absolute value) of output current of the signal generator 882 becomes maximum.